The present invention relates to a fluorocarbon compound-hydrogen storage alloy composite prepared by coating the surface of hydrogen storage alloy particles with a plated metal film containing a fine particle or particles of a water-repellent fluorocarbon compound, and a method of manufacturing the same. 2. Description of the Prior Art
With the recent spread of portable communication equipment and cordless equipment, the demand for secondary batteries, such as nickel-cadmium batteries, has drastically increased in order to realize reduction in size and weight of such equipment. From the viewpoints of environmental preservation of the globe and recycle of the resource, collection of used nickel-cadmium batteries becomes a social issue. Nickel-metal hydride batteries using a hydrogen storage alloy as a negative electrode have been noted as clean and high performance secondary batteries with a high energy density.
The negative electrode using a hydrogen storage alloy absorbs and releases hydrogen during the charging and discharging processes. In the charging process, the hydrogen storage alloy absorbs hydrogen while a hydride of the alloy absorbs an oxygen gas generated from the positive electrode. It is thus required that three phase zones comprising the solid phase, the liquid phase, and the gas phase exist in the electrode in a well balanced manner. In actual fact, however, the part in contact with the gas phase decreases, and the efficiency of the gas absorbing reaction is thereby lowered. Especially in the rapid charging process, the internal pressure of the battery abnormally increases and the electrolyte as well as the oxygen gas is released from the battery, which results in deteriorating the discharge capacity and the charge-discharge characteristics of the battery.
Conventionally, repeated charging and discharging operation increases the thickness of oxides or hydroxides formed on the surface of the alloy and enhances the contact resistance of alloy particles, thereby preventing the smooth progress of electrode reaction and deteriorating the performance of the battery.
The hydrogen storage alloy repeats expansion and contraction of volume by approximately 10 to 25%, due to absorption and desorption of hydrogen. The progress of charge-discharge cycle accordingly causes reduction of the particle size of the alloy, which results in undesirable release of fine alloy particles from the electrode.
One proposed countermeasure against the above problem includes mixing a polyfluorocarbon binder with hydrogen storage alloy particles plated with copper, molding the resultant mixture by cold press, and hot pressing the mold together with nickel meshes as a current collector at 300.degree. C. to yield an alloy negative electrode. This technique, however, has difficulty in homogeneously mixing the binder with the alloy particles. The excess amount of the binder undesirably increases the contact resistance of the alloy particles, whereas the insufficient amount results in poor formation of the gas phase, with a failure of attaining the required performance.
Strongly demanded is thus development of a highly reliable hydrogen storage alloy electrode which can be charged rapidly and endures repeated charging and discharging operation.